Use inline type hints in tests/ (#10350)

This PR is tantamount to running:

    python3.8 -m com2ann -v 6 tests/

(com2ann requires python 3.8 to run)

tests/storage/test_background_update.py

@@ -7,9 +7,7 @@ from tests import unittest
 
 class BackgroundUpdateTestCase(unittest.HomeserverTestCase):
     def prepare(self, reactor, clock, homeserver):
-        self.updates = (
-            self.hs.get_datastore().db_pool.updates
-        )  # type: BackgroundUpdater
+        self.updates: BackgroundUpdater = self.hs.get_datastore().db_pool.updates
         # the base test class should have run the real bg updates for us
         self.assertTrue(
             self.get_success(self.updates.has_completed_background_updates())